(1) Field of the Invention
This invention relates to a cadmium non-sintered negative electrode for use in an alkaline storage cell like a nickel-cadmium storage cell and its manufacturing method.
(2) Description of the Prior Art
As a cadmium negative electrode for use in an alkaline storage cell like a nickel-cadmium storage cell, a paste-type cadmium non-sintered negative electrode is widely used because it is manufactured easily and at a low cost. This type of cadmium negative electrode is produced by kneading a cadmium active material like powdered cadmium oxide or powdered cadmium hydroxide with a liquid binder to obtain a paste, coating the paste on a conductive substrate and drying it. However, this type of negative electrode has a problem that a cell employing it has a shorter cycle life than a cell employing a cadmium sintered negative electrode. This is attributed to the following two points (a) and (b):
(a) Capacity deterioration of the cadmium negative electrode.
It is known that a cadmium non-sintered negative electrode produced in the above method deteriorates its capacity in the course of charge-discharge cycles.
To solve this problem, U.S. Pat. No. 3,870,562 has proposed manufacturing a cadmium non-sintered negative electrode by mixing cadmium oxide, metal cadmium comprising globe grains having an average grain size of 3 to 12 .mu.m, and 0.1 to 5% of nickel hydroxide. The above patent says the above manufacturing method can restrict capacity deterioration which is caused by inactivation of the cadmium active material accompanied by charge-discharge cycles.
However, a residual capacity ratio of a cell which has been left for a while, namely storage characteristic and cycle characteristic of a cell vary depending on the grain shape and grain size of added nickel hydroxide.
(b) Leak of electrolyte caused by pressure valve operation
The paste including the above active material is soft. Therefore, when the negative electrode plate coated with the paste is wound together with a positive electrode and a separator by a pressure roller or the like to make an electrode assembly, the outermost end of the negative electrode of the assembly is given more pressure than its innermost end, whereby the porosity of the negative electrode plate is reduced. If the negative electrode plate has not been subjected to electrochemical formation when employed in a cell, the negative electrode plate retains a smaller amount of electrolyte, compared with if the same negative electrolyte plate has been subjected to electrochemical formation when employed in a cell. Therefore, in the former case, undischarged cadmium is accumulated in the cadmium negative electrode in the course of charge-discharge cycles. Accordingly, the porosity of the negative electrode is increased, whereby the amount of electrolyte retained in the negative electrode is increased. As a result, the amount of electrolyte retained in the separator is reduced, and so the cycle characteristic is lowered.
To solve this problem, another method has been proposed in which the above paste is coated on a conductive substrate to obtain an electrode plate and this plate is prehydrated in an aqueous solution of alkali. According to this method of chemically prehydrating cadmium oxide, cadmium hydroxide formed of large crystals is obtained. Therefore, comparatively large holes are uniformly formed in the paste-type active material and also the mechanical strength of the negative electrode plate is increased. Accordingly, even if the negative electrode plate is wound around by a pressure roller to make an electrode assembly, its porosity is kept uniformly high. If sodium hydroxide is used as an aqueous solution of alkali for prehydration, the obtained cadmium hydroxide is formed of .gamma.-type crystals, which make the cell easy to charge. This means that generation of hydrogen gas from the negative electrode plate is restricted, and so the inner pressure of the cell is prevented from increasing.
Though prehydration in an aqueous solution of sodium hydroxide takes time, it can be shortened by raising the temperature of the solution of sodium hydroxide.
However, prehydration is conducted in too a high temperature results in obtaining cadmium hydroxide formed of .beta.-type crystals instead of .gamma.-type ones. Since .beta.-type crystals are large enough to make the cell hard to charge, hydrogen gas is easily generated and so the inner pressure of the cell is increased. As a result, the pressure valve is operated to leak the electrolyte and then deteriorate the cell cycle characteristic.